Typical thick film resistors comprise a patterned layer (film) of a suitable resistive material of between 0.2 and 2.0 mils thick deposited on an insulating substrate. A thick film resistor is typically formed by first printing an appropriately patterned layer of suitably wet composition, usually an ink or paste, on an appropriate insulating substrate, generally an alumina ceramic board. Patterning is usually controlled by silk screen printing. The resistive composition is thereafter dried and fired (heated) to form the resistor.
Typically, printed circuit boards employ a first composition of inks or pastes for forming high resistance resistors and a different second composition for forming lower resistance resistors.
The resistance R of a particular resistor composition is generally tailored by adjusting its aspect ratio, L/W, where L is the length and W the width of the resistive film. R increases directly with L and inversely with W. R also increases inversely with the thickness of the film, but the thickness is generally kept uniform to facilitate processing. By appropriately tailoring the aspect ratio and by the use of high and low resistivity compositions of resistive material, a very wide range of resistances for the thick film resistors is achieved. Additional compositions can be used to fill in or extend the range, if necessary.
The printed circuit board typically supports a number of circuit elements (components) that are interconnected into a desired electrical circuit by an appropriately configured conductive film that is printed on the circuit board in an analogous fashion.
Copper (or a copper-rich alloy) is typically used on many printed circuit boards as the conductive film that interconnects the circuit elements since copper is of low cost, conducts well, resists migration and is easily soldered to. The copper film typically used is printed as a suspension of copper particles which is best fired in a nitrogen atmosphere to sinter the copper particles without oxidation into a good conductor. Materials typically preferred for forming thick film resistors are best fired in air. One problem is that copper films that have been formed by firing in nitrogen lose many of their favorable properties if subsequently refired in air.
One proposed solution to this problem is to first fire the thick film resistors in air and then to add copper conductors to the ends of the thick film resistors so as to make electrical contacts thereto. The copper conductors are then fired in nitrogen. This technique, in which the thick film resistors are formed first and their connections (terminations) are provided later, is generally known as "post termination". A problem with this type of post termination is that copper conductors generally make poor (high resistance) and non-uniform interfaces with the resistive film materials used in high performance thick film resistors, such as those based on a ruthenium compound (RuO.sub.2 or Bi.sub.2 Ru.sub.2 O.sub.7) for resistances above about 2,500 ohms.
A related concern is that this interface problem makes it difficult to accurately measure the values of such high value resistors using conventional copper-beryllium probes. Such measurements are usually important to maintain good quality control during the manufacture of printed circuit boards.
It is desirable to have a printed circuit board with high resistance thick film resistors and copper interconnections in which there is a well controlled, relatively low, and relatively uniform, contact resistance between the copper and the resistors.